1. Fields of the invention
The present invention relates to a tactile sensor, especially to a flexible piezoelectric tactile sensor that is applied to robots for control of actions such as grasping objects and motions. The tactile sensor can also be used in entertainment robots such as AIBO™ robotic pets designed and manufactured by SONY, as a communication interface between people and robotic pets.
1. In the information technology and computer field, the tactile sensors combined with monitors, used as touch input devices, have been broadly used in touch panels of Tablet personal computers (PC) and personal digital assistant (PDA), fingerprint recognition systems and virtual reality devices.2. In the manufacturing automation, the tactile sensor is used for equipment calibration and inspection/detection device of product design. For example, the force distribution of the contact between tread patterns and the ground is obtained by the tactile sensor. Thus a tire providing better traction is designed and produced.3. In the biomedical field, a hot device-smart skin is a typical application of the tactile sensor. As to a remote palpation system with tactile sensors developed by the research team of Professor R. Howe (Harvard Univ.), the device increases safety and reliability in detection of breast and prostate tumors.4. In the wireless physiological monitoring, the flexible tactile sensor has a great potential in development and production of wrist mounted or patch/disposable physiological monitors for monitoring respiration, heartbeat and pulse. Thus it is worth to invest such technology.
2. Descriptions of Related Art
Most of conventional piezoelectric tactile sensors are PVDF piezoelectric films with a silver electrode layer on both sides thereof. The silver electrode layer is patterned to form structural electrodes. However, the cost of the PVDF piezoelectric film with a silver electrode layer on both sides thereof is quite high and this dramatically increases the manufacturing cost of the sensor. Moreover, the patterning of the silver electrode layer is unable to be processed by conventional printed circuit board (PCB) manufacturing processes and the related manufacturing equipments are also quite expensive. Furthermore, traditional tactile sensors are used for normal force measurement while research of tactile sensors related to lateral force or multi-axis force measurement is rare. The followings are prior arts of tactile sensors regarding multi-axis force, lateral force slippage measurement etc. Refer to Taiwanese Pat. No. 1283295, a tactile sensor is revealed. In order to detect a remote stress, the sensitivity of the stress measurement still needs to be improved. Thus at least one lower substrate, a piezoelectric layer or an upper substrate is used as a pressure boost layer. At least one protuberant part is arranged between the pressure boost layer and a lower electrode layer/or an upper electrode layer so as to reduce the surface area. Accordingly, a mechanical stress applied to the piezoelectric layer is increased. Therefore, a sensing signal is generated by the piezoelectric layer when the mechanical stress is applied to the upper electrode layer. Refer to U.S. Pat. No. 5,871,248, a robot gripper is revealed. Without prior knowledge of the object's weight, size, or dimensions, the robot gripper has the capacity to handle various objects, used to apply enough gripping force to lift the object without slippage. The robot gripper includes flexible membranes that are filled with compressible fluid therein. When the gripper surfaces simultaneously apply compression and lifting forces to the object, a shear force will be generated which will distort the material. When the distortion begins to decrease, it will be safe to lift the object. The distortion is monitored by capacitance shear force sensors. Refer to U.S. Pat. RE37065, a triaxial force sensor is disclosed. The sensor includes a hemispherical target supported by a compliant element and ultrasonic transducers. The target is displaced by sufficient force applied to elastically deform the compliant element, which displacement alters the transit times of ultrasonic signals from the transducers which are reflected from the hemispherical target. The force is determined from the transit times, the deformation response of the compliant element etc. Refer to U.S. Pat. No. 4,745,812, a sensitive high resolution microminiature tactile sensor comprises an array of micromachined bossed silicon transducer cells and a thin polymer layer protectively covering the transducer cell array. The piezoresistive resistors are provided for measurement of transducer sensor strain resulting from loading applied to the tactile sensor so as to learn load magnitude and direction of the force. Thus it is an important issue to develop a piezoelectric tactile sensor that detects both multi-axis force and material force with reduced manufacturing cost from the view of structure, material and manufacturing processes.